Megasonic cleaning efficiency using auto-tuning of a RF generator at constant maximum efficiency

ABSTRACT

A system and method of cleaning a substrate includes a megasonic chamber that includes a transducer and a substrate. The transducer is being oriented toward the substrate. A variable distance d separates the transducer and the substrate. The system also includes a dynamically adjustable RF generator that has an output coupled to the transducer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of claims priority from U.S. patentapplication Ser. No. 10/360,322 filed on Feb. 6, 2003 and issued as U.S.Pat. No. 6,995,067 on Feb. 7, 2006, and entitled “Improved MegasonicCleaning Efficiency Using Auto-Tuning of an RF Generator at ConstantMaximum Efficiency,” which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to systems and methods of tuningan RF generator, and more particularly, to methods and systems forautomatically tuning an RF generator for a substrate cleaning system.

2. Description of the Related Art

The use of acoustic energy is a highly advanced, non-contact, cleaningtechnology for removing small-particles from substrates such assemiconductor wafers in various states of fabrication, flat paneldisplays, micro-electro-mechanical systems (MEMS),micro-opto-electro-mechanical systems (MOEMS), and the like. Thecleaning process typically involves the propagation of acoustic energythrough a liquid medium to remove particles from, and clean, a surfaceof a substrate. The megasonic energy is typically propagated in afrequency range of about 700 kHz (0.7 Megahertz (MHz)) to about 1.0 MHz,inclusive. The liquid medium can be deionized water or any one or moreof several substrate cleaning chemicals and combinations thereof. Thepropagation of acoustic energy through a liquid medium achievesnon-contact substrate cleaning chiefly through the formation andcollapse of bubbles from dissolved gases in the liquid medium, hereinreferred to as cavitation, microstreaming, and chemical reactionenhancement when chemicals are used as the liquid medium throughimproved mass transport, or providing activation energy to facilitatethe chemical reactions.

FIG. 1A is a diagram of a typical batch substrate cleaning system 10.FIG. 1B is a top view of the batch substrate cleaning system 10. A tank11 is filled with a cleaning solution 16 such as deionized water orother substrate cleaning chemicals. A substrate carrier 12, typically acassette of substrates, holds a batch of substrates 14 to be cleaned.One or more transducers 18A, 18B, 18C generate the emitted acousticenergy 15 that is propagated through the cleaning solution 16. Therelative location and distance between the substrates 14 and thetransducers 18A, 18B and 18C are typically approximately constant fromone batch of substrates 14 to another through use of locating fixtures19A, 19B that contact and locate the carrier 12.

The emitted energy 15, with or without appropriate chemistry to controlparticle re-adhesion, achieves substrate cleaning through cavitation,acoustic streaming, and enhanced mass transport if cleaning chemicalsare used. A batch substrate cleaning process typically requires lengthyprocessing times, and also can consume excessive volumes of cleaningchemicals 16. Additionally, consistency and substrate-to-substratecontrol are difficult to achieve. Such conditions as “shadowing” and“hot spots” are common in batch, and other, substrate megasonicprocesses. Shadowing occurs due to reflection and/or constructive anddestructive interference of emitted energy 15, and is compounded withthe additional substrate surface area of multiple substrates 14, wallsof the process tank etc. The occurrence of hot spots, primarily theresult of constructive interference due to the use of multipletransducers and to reflection, can also increase with additionalmultiple-substrate surface areas. These issues problems are typicallyaddressed by depending on the averaging effects of the multiplereflections of the acoustic energy on the substrate, which can lead to alower average power to the substrate surfaces. To compensate for thelower average power, and provide effective cleaning and particleremoval, power to the transducers is increased, thereby increasing theemitted energy 15 and increasing cavitation and acoustic streaming,which thereby increases the cleaning effectiveness. Additionally,pulsing the multiple transducer arrays 18A, 18B and 18C is used (i.e.providing a duty cycle such as turning the transducers on for 20 ms, andthen off for 10 ms. The transducers 18A, 18B and 18C can also beoperated out of phase (e.g., activated sequentially) to reduce compoundreflections and interference.

FIG. 1C is a prior art, schematic 30 of an RF supply to supply one ormore of the transducers 18A, 18B, 18C. An adjustable voltage controlledoscillator (VCO) 32 outputs a signal 33, at a selected frequency, to anRF generator 34. The RF generator 34 amplifies the signal 33 to producea signal 35 with an increased power. The signal 35 is output to thetransducer 18B. A power sensor 36 monitors the signal 35. The transducer18B outputs emitted energy 15.

The precise impedance of the transducer 18B can vary depending on manyvariables such as the number, size and spacing of substrates 14 in thecarrier 12 and the distance between the substrates 14 and the transducer18B. The precise impedance of the transducer 18B can also vary as thetransducer 18B ages through repeated usage. By way of example, ifsignals 33, 35 have a frequency of about 1 MHz, the wavelength is about1.5 mm (0.060 inches) in a deionized water medium such as the cleaningsolution 16. As a result, referring again to FIG. 1A, if the location ofthe substrates 14 and carrier 12 is off by as little as about 0.5 mm(0.020 inches) or even less, the impedance of the transducer 18B canvary substantially.

Adjusting the frequency of the VCO can adjust the impedance of thetransducer 18B by varying the frequency and therefore the wavelength ofthe signals 33, 35 and the emitted energy 15. Typically, a carrier 12that is loaded with substrates 14 is placed in the tank 11 and the VCO32 is adjusted to change the frequency of the signals 33, 35 and theemitted energy 15 until the impedance of the transducer 18B is matched,as indicated by a minimum value of a reflected signal 38 that isdetected by the power meter 36. Once the VCO 32 has been adjusted toachieve the minimum reflected signal 38, the VCO 32 is typically notadjusted again unless significant repairs or maintenance are performedon the substrate cleaning system 10.

When the transducer 18B impedance is not matched, a portion of theemitted energy 17 (i.e., waves) emitted from the transducer 18B isreflected back toward the transducer 18B. On the surface of thetransducer 18B, the reflected energy 17 can interfere with the emittedenergy 15 causing constructive and destructive interference. Thedestructive interference reduces the effective cleaning power of theemitted energy 15 because a portion of the emitted energy 15 iseffectively cancelled out by the reflected energy 17. As a result, theRF generator 34 efficiency is reduced.

The constructive interference can cause excess energy that can cause hotspots on the surfaces of the substrates 14 being cleaned. The hot spotscan exceed an energy threshold of the substrates 14 and can damage thesubstrates 14. FIG. 1D is a typical transducer 18B. FIG. 1E is a graph100 of the energy distribution across the transducer 18B. Curve 102 is acurve of the energy emitted across the transducer 18B in the x-axis.Curve 104 is a curve of the energy emitted across the transducer 18B inthe y-axis. Curve 120 is a curve of the composite energy emitted acrossthe transducer 18B in both the x-axis and the y-axis. The compositeenergy emitted across the transducer 18B in both the x-axis and they-axis typically can vary between curve 120 and curve 122 as the knownvariations (e.g., location of the substrates, aging of the transducer,and wobble of a rotating substrate relative to the transducer etc.)cause the impedance of the transducer 18B to vary. A threshold energylevel T is the damage threshold to the substrate(s) 14. Typically, themaximum power of the RF signal 35 and the resulting emitted energy 15output by the transducer 18B is reduced to a level such that the maximumconstructive interference results in a peak magnitude (i.e., peaks incurve 120) of less than the energy threshold T of the substrates 14 soas to prevent damage to the substrate 14. However, the reduced power ofthe RF signal 35 and the emitted energy 15 increases the cleaningprocess time required to achieve the desired cleaning result. In someinstances, the reduced power of the signal 35 and the emitted energy 15is insufficient to remove the some of the targeted particles from thesubstrates 14. As shown, the effective emitted energy can vary to a muchlower level (represented by valleys in curve 122) such that theeffectiveness of the cleaning process is severely impacted because theeffective energy is so low (about 3) and therefore results in an energywindow that extends from about 3 to about 17 as shown on the energyscale.

The transducer 18B is typically a piezoelectric device such as acrystal. The constructive and destructive interference caused by thereflected energy 17 can also impart a force to the surface of thetransducer 18B sufficient to cause the transducer 18B to produce acorresponding reflected signal 38. The power sensor 36 can detect thereflected signal 38 that is reflected from the transducer 18B toward theRF generator 34. The reflected signal 38 can constructively ordestructively interfere with the signal 35 output from the RF generator34 to further reduce the efficiency of the RF generator 34.

In view of the foregoing, there is a need for an improved megasoniccleaning system that provides increased efficiency of the RF generatorand a reduced energy window of the emitted acoustic energy and reducesthe probability of substrate damage.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention fills these needs by providing adynamically cleaning system that uses an automatically tuned RFgenerator that is constantly tuned to the maintain resonance of thetransducer and the emitted energy from the transducer. It should beappreciated that the present invention can be implemented in numerousways, including as a process, an apparatus, a system, computer readablemedia, or a device. Several inventive embodiments of the presentinvention are described below.

One embodiment includes a method of cleaning a substrate that includesapplying an RF signal at a frequency f to a transducer. The transducerbeing oriented toward the substrate such that the transducer emits anacoustic energy at the frequency f toward the substrate. The substrateis moved relative to the transducer. The RF signal is dynamicallyadjusted to maintain a resonance of the acoustic energy.

Dynamically adjusting frequency f can include automatically adjustingthe frequency f for each cycle of the RF signal.

Moving the substrate relative to the transducer can include rotating thesubstrate.

The substrate can also be submerged in a cleaning solution. The cleaningsolution can be deionized water. The cleaning solution can include oneor more of a plurality of cleaning chemicals.

Dynamically adjusting the RF signal to maintain the resonance of theacoustic energy can include maintaining a constant voltage of the RFsignal applied to the transducer.

An RF generator can apply the RF signal to the transducer andmaintaining a constant voltage of the RF signal applied to thetransducer can include measuring a first voltage of the RF signal,comparing the first voltage to a desired set point voltage, andinputting a control signal to a variable DC power supply so as to adjustan output voltage of the variable DC power supply, the variable DC powersupply supplying DC power to the RF generator.

Dynamically adjusting the RF signal to maintain the resonance of theacoustic energy can include dynamically adjusting a frequency f of theRF signal applied to the transducer.

An RF generator can apply the RF signal to the transducer anddynamically adjusting the frequency f of the RF signal applied to thetransducer can include measuring a supply voltage applied to the RFgenerator, measuring a peak voltage across an output amplifier includedin the RF generator, producing a frequency control signal when the peakvoltage is not equal to a selected ratio of the supply voltage, andapplying the frequency control signal to a frequency control input of anoscillator that generates the RF signal.

An RF generator can apply the RF signal to the transducer anddynamically adjusting the frequency f of the RF signal applied to thetransducer can include inputting an RF input signal from an oscillatorthe RF generator and amplifying the RF signal in the RF generator. Afirst phase of an input voltage of the RF input signal is measured, asecond phase of a voltage of the RF signal output from the RF generatoris measured. A frequency control signal is produced when the first phaseis not equal to the second phase. The frequency control signal isapplied to a frequency control input of the oscillator.

Another embodiment includes a system for cleaning a substrate includes acleaning chamber that includes a transducer and a substrate. Thetransducer being oriented toward the substrate. A variable distance dseparates the transducer and the substrate. The system also includes adynamically adjustable RF generator that has an output coupled to thetransducer and a feedback circuit coupled to a control input of theadjustable RF generator.

The substrate can be rotated. The distance d can vary about ½ wavelengthof an RF signal output from the RF generator as the substrate isrotated.

The dynamically adjustable RF generator can include a variable DC powersupply having a control input and a DC output coupled to the RFgenerator. The feedback circuit can include a first comparator thatincludes a first input coupled to a set point control signal, a secondinput coupled to the RF generator RF output, and a control signal outputcoupled to the control input of the adjustable RF generator. The controlinput includes a voltage control input on the variable DC power supply.

The dynamically adjustable RF generator can include an oscillator, anoutput amplifier coupled to the oscillator output and a load network.The oscillator has a control signal input and an RF signal output. Theload network coupled between an output of the output amplifier and theoutput of the RF generator. The feedback circuit can include a peakvoltage detector, and a second comparator. The peak voltage detector canbe coupled across the output amplifier. The second comparator includes athird input coupled to an output of the variable DC power supply, afourth input coupled to an output of the peak voltage detector, and asecond comparator output coupled to the control input of the adjustableRF generator. The control input can include the oscillator controlsignal input.

The dynamically adjustable RF generator can include an oscillator and anRF generator input coupled to the oscillator RF signal output. Theoscillator having a frequency control input and an RF signal output. Thefeedback circuit can include a voltage phase detector. The voltage phasedetector can include a first phase input coupled to the RF signal outputof the oscillator, a second phase input coupled to the RF generatoroutput, and a frequency control signal output coupled to the controlinput of the adjustable RF generator. The control input can include theoscillator frequency control voltage input.

The dynamically adjustable RF generator can include a supply voltagesource, an oscillator having a control signal input and an RF signaloutput, an output amplifier coupled to the oscillator output, a loadnetwork coupled between an output of the output amplifier and the outputof the RF generator. The feedback circuit can include a peak voltagedetector coupled across the output amplifier, and a comparator circuit.The comparator circuit can include a first input coupled to the supplyvoltage source, a second input coupled to an output of the peak voltagedetector, and a comparator output coupled to the control input of theadjustable RF generator. The control input can include the oscillatorcontrol signal input.

The dynamically adjustable RF generator can include an oscillator, an RFgenerator input coupled to the oscillator RF signal output, and thefeedback circuit can include a voltage phase detector. The oscillatorhas a frequency control input and an RF signal output. The voltage phasedetector that includes a first phase input coupled to the RF signaloutput of the oscillator, a second phase input coupled to the RFgenerator output, and a frequency control signal output coupled to thecontrol input of the adjustable RF generator. The control input caninclude the oscillator frequency control voltage input.

The transducer can include two or more transducers. The dynamicallyadjustable RF generator can include two or more dynamically adjustableRF generators each having a respective output coupled to one of the twoor more transducers.

The transducer can include a first transducer oriented toward an activesurface of the substrate and a second transducer oriented toward anon-active side of the substrate.

The present invention provides the advantage of significantly reducedcleaning processing time because the higher power acoustic energy can beused without damaging the substrate being cleaned.

The present invention also reduces the number of substrates damaged dueto excess acoustic energy being applied to the substrate.

The auto-tuned RF generator also automatically adjusts forprocess-induced impedance changes such as different cleaningchemistries, different locations of the substrate, different processtank configurations etc, thereby providing a more flexible and robustcleaning process.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings, andlike reference numerals designate like structural elements.

FIG. 1A is a diagram of a typical batch substrate cleaning system.

FIG. 1B is a top view of the batch substrate cleaning system.

FIG. 1C is a prior art, schematic of an RF supply to supply one or moreof the transducers.

FIG. 1D is a typical transducer 18B.

FIG. 1E is a graph of the energy distribution across the transducer.

FIGS. 2A and 2B show a dynamic, single substrate cleaning system, inaccordance with one embodiment of the present invention.

FIG. 2C is a flowchart of the method operations of an auto-tuning RFgenerator system used in a megasonic cleaning system, such as describedin FIGS. 2A and 2B above, in accordance with one embodiment of thepresent invention.

FIG. 3 is a block diagram of an auto-tuning RF generator system inaccordance with one embodiment of the present invention.

FIG. 4 is a flowchart of the method operations of the auto-tuning RFgenerator system while the RF generator is supplying an RF signal to thetransducer, in accordance with one embodiment of the present invention.

FIG. 5A is a schematic diagram of the peak V_(ds) detector in accordancewith one embodiment of the present invention.

FIG. 5 B is a graph of waveforms of the peak voltage (V_(ds)) detectedby the peak voltage detector, in accordance with one embodiment of thepresent invention.

FIG. 6 is a block diagram of an auto-tuning RF generator systemaccording to one embodiment of the present invention.

FIG. 7 is a flowchart of the method operations of the auto-tuning RFgenerator system according to one embodiment of the present invention.

FIGS. 8A-8C show graphs of three examples of the relationships betweenphase P1 and phase P2 in accordance with one embodiment of the presentinvention.

FIG. 9 is a block diagram of an auto-tuning RF generator systemaccording to one embodiment of the present invention.

FIG. 10 is a flowchart of the method operations of the auto-tuning RFgenerator system, in accordance with one embodiment of the presentinvention.

FIG. 11 is a diagram of a megasonic module in accordance with oneembodiment of the present invention.

FIG. 12 is a graph of the energy distribution across the transducer inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Several exemplary embodiments for an acoustic energy cleaning systemthat automatically adjusts RF signal applied to the transducer formaximum efficiency will now be described. It will be apparent to thoseskilled in the art that the present invention may be practiced withoutsome or all of the specific details set forth herein.

As described above, it is very important to increase the cleaningeffectiveness, efficiencies and throughput rate of substrate cleaningsystems, while reducing probability of damage to the substrate. Theserequirements are exacerbated by the continuously shrinking device sizesand the fact that many cleaning systems are evolving to single substratecleaning systems.

FIGS. 2A and 2B show a dynamic, single substrate cleaning system 200, inaccordance with one embodiment of the present invention. FIG. 2A shows aside view of the dynamic, single substrate cleaning system 200. FIG. 2Bshows a top view of the dynamic, single substrate cleaning system 200.The substrate 202 is immersed in cleaning solution 204 contained withina cleaning chamber 206. The cleaning solution 204 can be deionized water(DI water) or other cleaning chemistries that are well known in the artand combinations thereof.

The substrate 202 is substantially circular and is held by three or moreedge rollers 208A, 208B, 208C (or similar edge holding devices) so thatthe substrate 202 can be rotated (e.g., in direction 209A) as thecleaning process is applied to the substrate 202. One or more of thethree edge rollers 208A, 208B, and 208C can be driven (e.g. in direction209B) so as to rotate the substrate 202 in direction 209A. The substrate202 can be rotated at a rate of up to about 500 RPM.

A transducer 210 is also included as part of the cleaning chamber 206.The transducer 210 can be a piezoelectric device such as a crystal thatcan convert an RF signal 220 to acoustic energy 214 emitted into thecleaning solution 204. The transducer 210 can be composed ofpiezoelectric material such as piezoelectric ceramic, lead zirconiumtintanate, piezoelectric quartz, gallium phosphate wherein thepiezoelectric material is bonded to a resonator such as ceramic, siliconcarbide, stainless steel or aluminum, or quartz.

As shown in FIG. 2B, the transducer 210 can be significantly smallerthan the substrate 202. Smaller transducers can be manufactured moreinexpensively and can also offer improved control over the smaller areaof the substrate 202 that the emitted energy 214 emitted from thesmaller transducer 210 impacts. The active surface 218 (i.e., thesurface having the active devices thereon) of the substrate 202 istypically facing the transducer 210. However, in some embodiments theactive surface 218 can be on the side of the substrate 202 opposite thetransducer 210.

The three edge rollers 208A, 208B, 208C hold the substrate 202approximately a fixed distance d1 from the transducer 210 as thesubstrate 202 rotates past the transducer 210. Distance d1 can be withina range of only a few millimeters to up to about 100 mm or more. Thedistance d1 is selected as a distance that matches the impedance of thetransducer 210. In one embodiment the distance d1 is selected as aresonant distance for the frequency of the emitted energy 214.Alternatively, the frequency of the emitted energy 214 can be selectedso that the distance d1 is a resonant distance. In either embodiment, atresonance, the minimum reflected energy 216 is reflected from thesubstrate 202 back toward the transducer 210. As described above, thereflected energy 216 can interfere with the emitted energy 214 which candecrease the power efficiency of the RF signal 220 and can causedecreased cleaning effectiveness (e.g., interference patterns) on thesubstrate 210.

However, the substrate 202 can “wobble” somewhat such that the distancebetween the substrate 202 and the transducer 210 can vary between thefirst distance d1 to a second distance d2 as the substrate 202 rotatespast the transducer 210. The difference between the first distance d1and the second distance d2 can be up to about 0.5 mm (0.020 inches) oreven greater. While improved edge rollers 208A, 208B, 208C and othersimilar technologies may be able to hold the substrate 202 a moreconsistent distance d1 from the transducer 210, the improved edgerollers cannot guarantee an absolute constant distance d1 and thereforevariations in the distance d1 can still occur. Further, the distancebetween the substrate 202 and the transducer 210 can vary for otherreasons as well (e.g. placement of the substrate 202 within the edgerollers 208A, 208B, 208C, etc.). As will be described in more detailbelow, the variation in the distance between the substrate 202 and thetransducer 210 can severely impact performance and efficiency of thecleaning system 200.

The transducer 210 is coupled to an RF generator 212. FIG. 2C is aflowchart of the method operations 250 of an auto-tuning RF generatorsystem used in a megasonic cleaning system 200, such as described inFIGS. 2A and 2B above, in accordance with one embodiment of the presentinvention. In operation 255, the RF generator provides the RF signal 220to the transducer 210. The RF signal 220 can have a frequency of betweenabout 400 kHz to about 2 MHz but is typically between about 700 kHz toabout 1 MHz. The wavelength of the high frequency acoustic energy 214emitted from the transducer 210 is about 1.5 mm (0.060 inches) inlength, in the cleaning solution 204.

In operation 260, the distance to the target (e.g., substrate 202)varies as the target is moved, relative to the transducer 210. As thedistance d1 varies the amount of reflected energy 216 also variesbecause the emitted energy 214 is not always in resonance when thedistance d1 changes (i.e. the impedance of the transducer 210 ismismatched). In operation 270, the RF generator 212 is automatically anddynamically tuned so that the RF signal 220 is constantly tuned tocorrect for any impedance mismatches as the distance d1 changes.

Because a wavelength of the emitted energy 214 is about 1.5 mm (0.060inches), a movement of only 0.50 mm (0.020 inches) can cause asignificant impedance variation resulting in, for example, as much as a50% variation in voltage and power varying between about 25% and 100%.Without an auto-tuning RF generator to compensate for the variations ind1, the peak energy level of the emitted energy 214 must be reduced to alow enough value that the energy absorbing ability of the substrate 202(energy threshold) is not exceeded so as to prevent the peak emittedenergy 214 from damaging the substrate 202.

The auto-tuning RF generator 212 can be automatically tuned tocompensate for the variations in the distance d1 through varyingapproaches. In one embodiment, a peak voltage is detected so as tomaintain the RF generator 212 at an impedance optimized frequency of theRF signal 220. In another embodiment, the phase of the voltage ismaintained so as to produce an impedance optimized frequency of the RFsignal 220. In yet another embodiment, the power supply can be adjustedto impedance optimize the RF signal 220. The various embodiments canalso be used in combination within a single auto-tuning RF generatorsystem.

FIG. 3 is a block diagram of an auto-tuning RF generator system 300according to one embodiment of the present invention. The auto-tuning RFgenerator 302 provides a feedback control signal to the voltagecontrolled oscillator (VCO) 306 so as to adjust the frequency of a VCORF signal 310 output from the VCO 306. The VCO 306 can also be includedas part of the RF generator 302. A DC power supply 312 is included andprovides DC power for the amplification of the VCO RF signal 310 in theRF generator 302. The auto-tuning RF generator 302 includes an inductor314 in the input portion of the RF generator 302. One or more amplifiers320 that amplify the VCO RF signal 310 are also included in the RFgenerator 302.

In one embodiment, the amplifier 320 is a CMOS and the VCO RF signal 310is applied to a gate G. A drain D is coupled to DC bias rail 322 and asource S is coupled to a ground potential rail 324. A peak voltage drainto source (peak V_(ds)) detector 326 is coupled across the drain D andsource S terminals of the amplifier 320 so as to capture the peakvoltage drain to source of the amplifier 320.

The output of the amplifier 320 is coupled to an input of a class-E loadnetwork 330. The class-E load network 330 is a common device well knownin the art for performing large-scale impedance matching functionsbetween an RF source (i.e., RF generator 302) and an RF load (i.e.transducer 332). The class-E load network 330 typically includes a LCnetwork. An output of the class-E load network 330 is coupled to aninput to the transducer 332.

FIG. 4 is a flowchart of the method operations 400 of the auto-tuning RFgenerator system 300 while the RF generator 302 is supplying an RFsignal 220 to the transducer 332, in accordance with one embodiment ofthe present invention. In operation 405, the DC supply voltage ismeasured or detected by a comparator device 340. A voltage dividernetwork 342 can also be included to scale or reduce the amplitude of therespective voltage coupled to the comparator device 340 from the DCpower supply 312 to a level useable by the comparator device 340.Proportional, differential and integral controls can also be included inthe comparator device 340 so that the rate and amount of change in thecontrol signal can be selected.

In operation 410, the peak V_(ds) is detected by the peak V_(ds)detector 326 and applied to a second input of the comparator device 340.The peak V_(ds) detector 326 can also include circuitry to scale orreduce the amplitude of the voltage coupled to the comparator device 340from the peak V_(ds) detector 326 to a level useable by the comparatordevice 340.

By way of example, the DC power supply 312 may output 200 VDC and thecomparator device 340 is capable of comparing a 5 VDC signal, thereforethe voltage divider network 342 can scale DC power supply voltage from200 VDC to a voltage of 5 VDC that represents 200 VDC in the comparatordevice 340. Similarly, the peak V_(ds) detector 326 can also includescaling devices such as a voltage divider network so that the actualpeak V_(ds) voltage applied to the comparator device 340 is about 5 VDC.

In operation 415, the comparator device 340 compares the peak V_(ds) andthe DC supply voltage from the DC power supply 312. If the DC supplyvoltage is a desired ratio of the peak V_(ds), then no correction signalis output from the comparator device and the method operations continuein operation 405 above.

Alternatively, if the DC supply voltage is not a desired ratio of thepeak V_(ds), then the method operations continue in operation 420. Inoperation 420, a corresponding correction signal is output from thecomparator device 340 to the VCO 306 to adjust the frequency of the VCOoutput signal 310 and the method operations continue in operation 405above. The correction signal can adjust the frequency of the VCO RFsignal 310 to a higher or lower frequency as required.

The desired ratio of the DC supply voltage to the peak V_(ds), isdependant upon the particular values of the various components in the RFgenerator 302 and the transducer 332 and the system that may include theRF generator 302 and the transducer 332, such as the substrate cleaningsystem 200 of FIG. 2 above. In one embodiment, the desired ratio iswithin a range of about 3:1 to about 6:1, where the peak V_(ds) is alarger voltage than the DC supply voltage. In one embodiment the desiredratio is about 4:1 and more specifically about 3.6:1 where the peakV_(ds) is about equal to about a 3.6 multiple of the DC supply voltage.

FIG. 5A is a schematic diagram of the peak V_(ds) detector 326 inaccordance with one embodiment of the present invention. Seriallyconnected capacitors 502, 504 are coupled across the drain D and sourceS of the amplifier 320. A diode 506 is coupled in parallel withcapacitor 504. In operation, capacitor 502 couples the peak V_(ds) ofeach cycle of the amplified RF signal to capacitor 504. Capacitor 504stores the peak V_(ds) for each cycle of the amplified RF signal that isoutput from the amplifier 320. Diode 506 captures the peak V_(ds) andcouples the peak V_(ds) to the comparator device 340 via the peak V_(ds)terminal.

FIG. 5B is a graph 550 of waveforms of the peak voltage (V_(ds))detected by the peak voltage detector 326, in accordance with oneembodiment of the present invention. When the amplifier device 320 isconducting, the peak voltage detector 326 does not detect much voltagebecause there is little voltage drop across the amplifier 320. When theamplifier stops conducting, then the current stored in the inductors andcapacitors of the RF generator 302 and load network 330 is discharged,resulting in a voltage waveform 552, 554, 556 as detected by the peakvoltage detector 326. The amplifier 320 is designed such that as thevoltage across the amplifier 320 (V_(ds)) drops to zero, the amplifier320 begins to conduct thus creating a tuned amplification circuit. Thetuned amplification circuit is affected by any changes in resonance ofthe transducer 332 (e.g., any movement of the substrate 202 relative tothe transducer 332), which are reflected through the load network 330 tochange the detected waveform 552, 554, 556. When in resonance, theamplifier 320 acts as a well tuned class-E amplifier and the waveform554 occurs. When off resonance, the transducer 332 can have eithercapacitive or inductive reactance resulting in added capacitive orinductive reactance, which detunes the class-E load network 330. Thedetuned class-E load network 330 results in either waveform 552 or 556,having either a too high peak voltage V1 or too low peak voltage V3.

Through experimentation and calculation, it has been found that the peakvoltage (V_(ds)) is a function of the resonance of the transducer 332and the peak V_(ds) compared to the applied DC bias voltage has aresonant ratio that is a function of the components of the RF generatorcircuit 302. For example, in a typical RF generator, the ratio is about4:1 peak voltage as compared to the DC bias voltage from the DC powersupply, or restated, a peak V_(ds) of about 4 multiples of the biasvoltage from the DC power supply 312 indicates that the transducer 332is in resonance.

FIG. 6 is a block diagram of an auto-tuning RF generator system 600according to one embodiment of the present invention. A phase P1 of thevoltage of the RF signal 310 output from the VCO 306 is compared to aphase P2 of the voltage of the input to the transducer 332. If thevoltage phases P1 and P2 do not match, a correction signal is applied tothe frequency control input of the VCO 306. The RF generator system 600includes an RF generator 602. The RF generator 602 can be any type of RFgenerator known in the art. A phase detector 604 includes two inputs606, 608. The first and second inputs 606, 608 can also includerespective scaling circuits 610, 612 (e.g., voltage divider networks)that can scale the detected signals (e.g. phase P1 and phase P2) to alevel useable by the phase detector 604. The phase detector 604 can beany type of phase detector known in the art that can detect and comparethe phases of the respective input voltage signals. Prior art phasedetectors compared the phases of the voltage and current of the outputRF signal 220. Testing has shown that comparing the voltage phases P1and P2 can be accomplished more simply and easily and provide the neededsignal for adjusting the VCO 306 accordingly.

FIG. 7 is a flowchart of the method operations of the auto-tuning RFgenerator system 600 according to one embodiment of the presentinvention. In operation 705, an input RF signal 310 from the VCO 306 isapplied to the RF generator 602 and the RF generator 602 amplifies theinput RF signal 310 and couples the amplified RF signal 220 to thetransducer 332.

In operation 710, the first input 606 couples a first phase (P1) of thevoltage of the RF signal 310 output from the VCO 306 to the phasedetector 604. In operation 715 the second input 608 couples a secondphase (P2) of the voltage of the signal input to the transducer 332 tothe phase detector 604.

In operation 720, the phase detector compares phase P1 and phase P2 todetermine if the phase P1 matches phase P2. FIGS. 8A-8C show graphs ofthree examples of the relationships between phases P1 and P2, inaccordance with one embodiment of the present invention. In FIG. 8A,graph 800 shows phase P1 leads phase P2 (e.g., phase P1 peaks at time T1and phase P2 peaks at a subsequent time T2). This indicates that theimpedance of the transducer 332 is not matched and that the transducer332 is applying a reflected signal 222 into the RF generator 602.

In FIG. 8B, graph 820 shows phase P1 lags phase P2 (e.g., phase P2 peaksat time T1 and phase P1 peaks at a subsequent time T2). This indicatesthat the impedance of the transducer 332 is not matched and that thetransducer 332 is again applying a reflected signal 222 into the RFgenerator 602. The reflected signal 222 output by the transducer 332 canbe constructively or destructively interfering with the signal 220output from the RF generator 602.

In FIG. 8C, graph 850 shows phase P1 is equal to phase P2 (e.g., bothphase P1 and phase P2 peak at time T1). This indicates that theimpedance of the transducer 332 is matched and that the transducer 332is not applying any reflected signal into the RF generator 602.

If, in operation 720, phase P1 and phase P2 are equal, then the methodoperations continue (repeat) at operation 705. If, however, in operation720 phase P1 and phase P2 are not equal, then the method operationscontinue in operation 730. In operation 730, an appropriate controlsignal is applied to the frequency control input of the VCO 306 toadjust the frequency of the RF signal 310 accordingly, and the methodoperations continue (repeat) at operation 705. The control signalapplied to the frequency control input of the VCO 306 can adjust thefrequency to a higher frequency in response to a condition where phaseP1 leads phase P2. Alternatively, the control signal applied to thefrequency control input of the VCO 306 can adjust the frequency to alower frequency in response to a condition where phase P1 lags phase P2.

The auto-tuning RF generator system 600 can also include a controlamplifier 620 that can scale the control signal output by the phasedetector 604 to the correct signal level to control the VCO 306. Thecontrol amplifier 620 can also include a set point input so the controlamplifier 620 can combine the set point input and the control signalinput from the phase detector. In this manner a VCO RF signal 310 can beselected by the set point and then the control signal output by thephase detector 604 can automatically adjust the selected set point.

The systems and methods described in FIGS. 3 through 8C above canautomatically tune the RF generators 302, 602 at a very high correctionrate (e.g., at each cycle of the input RF signal 310 can cause asubsequent correction in the frequency of the RF signal 310 and theoutput RF signal 220). As a result, the frequency of the input RF signal310 can be corrected, for example, multiple times during each revolutionof the substrate 202 and thereby providing much more precise control ofthe acoustic energy 214 applied to the substrate 202.

By way of example, if the substrate 202 is being rotated 60 RPM (i.e. 1revolution per second) and the RF signal 310 is about 1 MHz, then thefrequency of the RF signal 310 can be adjusted about one million timesper second (i.e., once per microsecond) during each rotation of thesubstrate 202. This increased control of the acoustic energy 214 appliedto the substrate 202 means that the average energy can be very close tothe minimum energy valley and the maximum energy peak of the emittedenergy 214. Therefore a higher average energy can be applied to thesubstrate 202, which thereby allows a significantly reduced cleaningprocess time and improved cleaning effectiveness.

FIG. 9 is a block diagram of an auto-tuning RF generator system 900according to one embodiment of the present invention. The systemincludes a VCO 306 that is coupled to an input of an RF generator 602. Avariable DC power supply 902 is coupled to the RF generator 602 andprovides DC power for the RF generator to amplify the RF signal 310 fromthe VCO 306. The output of the RF generator 602 is coupled to thetransducer 332.

Typical prior art acoustic energy cleaning systems focus on maintaininga constant net power input to the transducer 332 (i.e., forward power ofRF signal 220 less reflected power of reflected signal 222). Throughexperimentation, it has been found that if the voltage of the RF signal220 is maintained as a constant voltage, then the amplitude of theemitter energy 214 output from the transducer 332 is substantiallyconstant. Further, maintaining the voltage of the RF signal 220 at aconstant level, below the energy threshold limit of the substrate 202protects the substrate from damage while also allowing a maximumacoustic energy 214 to be applied to the substrate 202.

FIG. 10 is a flowchart of the method operations of the auto-tuning RFgenerator system 900, in accordance with one embodiment of the presentinvention. In operation 1005, the RF generator 602 outputs an RF signalto the transducer 332. In operation 1010, a voltage of the RF signaloutput to the transducer 332 is measured and coupled to a comparator904.

In operation 1015, the comparator 904 compares the voltage of the RFsignal output from the RF generator 602 to a desired set point voltage.If the output voltage is equal to the desired set point voltage, themethod operations continue at operation 1010. Alternatively, if theoutput voltage is not equal to the set point voltage, the methodoperations continue in operation 1030.

In operation 1030, the comparator 904 outputs a control signal to acontrol input on the variable DC power supply 902. By way of example, ifthe output voltage is too high (i.e., greater than the desired set pointvoltage), then the control signal will reduce the DC voltage output fromthe variable DC power supply 902 thereby reducing the gain of theamplification that occurs within the RF generator 602, thereby reducingthe amplitude of the RF signal output by the RF generator 602.Proportional, differential and integral controls can also be included inthe comparator 904 so that the rate and amount of change in the controlsignal can be selected.

A scaling circuit 906 can also be included to scale the voltage outputfrom the RF generator 602 to a level more easily compared to the setpoint signal. By way of example, the scaling circuit 906 can scale a 200V RF signal to 5 V for comparison to a 5 V set point signal. The scalingcircuit 906 can include a voltage divider. The scaling circuit 906 canalso include a rectifier to rectify the voltage of RF signal 220 outputfrom the RF generator 602 to a DC voltage for comparison to a DC setpoint signal.

As described above the methods described in FIGS. 3 through 8C above canautomatically tune the RF generators 302, 602 at a very high correctionrate (e.g., once per a few cycles of the RF signal 310). Conversely, thesystem and method described in FIGS. 9 and 10 can also automaticallytune the RF generator 602 but at a slightly slower rate than asdescribed in FIGS. 3 through 8C but yet still faster than the likelychanges in impedance of the transducer 332 due to the motion of thesubstrate 202. The system and method described in FIGS. 9 and 10 issomewhat slower due in part to the hysteresis included in the variableDC power supply 902.

The system and method described in FIGS. 9 and 10 can be used incombination with one or more of the systems and methods described inFIGS. 3 through 8C above. As such, the system and method described inFIGS. 9 and 10 can used to provide a very broad range of tuning the RFgenerator to the dynamic resonance of the transducer 332, while thesystems and methods described in FIGS. 3 through 8C above can be used toprovide very fine control and adjustment of the tuning the RF generator.

FIG. 11 is a diagram of a megasonic module 1100 in accordance with oneembodiment of the present invention. The megasonic module 1100 can be amegasonic module, such as a the material described in commonly ownedU.S. patent application Ser. No. 10/259,023, entitled “MegasonicSubstrate Processing Module” which was filed on Sep. 26, 2002, which isincorporated by reference herein, in its entirety, for all purposes.

The megasonic module 1100 includes a substrate processing tank 1102(hereinafter referred to as tank 1102), and a tank lid 1104 (hereinafterreferred to as lid 1104). A lid megasonic transducer 1108 and a tankmegasonic transducer 1106 are positioned on lid 1104 and in tank 1102,respectively, and provide megasonic energy for simultaneously processingan active and a backside surface of a substrate 1110. A substrate 1110is positioned in drive wheels 1112, and secured in position withsubstrate stabilizing arm/wheel 1114. In one embodiment, the substratestabilizing arm/wheel 1114 is positioned with an actuator 1120 and apositioning rod 1122 to open and close the stabilizing arm/wheel 1114 toreceive, secure, and release a substrate 1110 to be processed in themegasonic module 1100. The lid 1104 can be positioned in an open or aclosed position with a actuator system (not shown) that raises andlowers lid 1104 while the tank 1102 remains stationary. Alternativelythe tank 1102 can be moved to mate with the lid 1104.

In one embodiment, substrate stabilizing arm/wheel 1114 is configured tosecure and support substrate 1110 in a horizontal orientation forprocessing, and to allow rotation of substrate 1110. In otherembodiments, substrate processing is performed with substrate 1110 in avertical orientation. Drive wheels 1112 contact a peripheral edge ofsubstrate 1110 and rotate substrate 1110 during processing. Substratestabilizing arm/wheel 1114 can include a freely spinning wheel to allowfor substrate 1110 rotation while supporting substrate 1110 in ahorizontal orientation.

Once the substrate 1110 is placed in the tank 1102, the tank 1102 isthen filled with processing fluid including deionized (DI) water, orprocessing chemicals as desired. Once the closed megasonic module 1100is filled with desired processing fluid, and substrate 1110 is immersedtherein, megasonic processing of substrate 1110 is accomplished by tankmegasonic transducer 1106 directing megasonic energy against the surfaceof substrate 1110 facing the tank megasonic transducer 1106, and by lidmegasonic transducer 1108 directing megasonic energy against the surfaceof substrate 1110 facing the lid megasonic transducer 1108. Withsubstrate 1110 submerged in processing chemicals, drive wheels 1112rotate substrate 1110 to ensure complete and uniform processing acrossthe entire surface of both the active and backside surfaces of substrate1110. In one embodiment, drive motor 1116 is provided to drive the drivewheels 1112 via a mechanical coupling 1118 (e.g., drive belt, gears,sprocket and chain, etc.).

An auto-tuning RF generator system as described in FIGS. 3-10 above canbe coupled to one or both of the lid transducer 1108 and tank transducer1106 so that the respective transducers 1108, 1106 are constantly andautomatically tuned for the dynamic impedance of the respectivetransducers 1108, 1106 as the substrate 1110 is rotated.

FIG. 12 is a graph 1200 of the energy distribution across the transducerin accordance with one embodiment of the present invention. Incomparison with the prior art energy window shown by curves 120 and 122,an auto-tuning RF generator can result in a much narrower energy window1202 between curve 1210 and curve 1212. Since the energy window 1202 ismuch narrower, then the energy window can be shifted upward closer tothe energy threshold T of the substrate and thereby provide a moreeffective acoustic energy cleaning process.

As used herein in connection with the description of the invention, theterm “about” means+/−10%. By way of example, the phrase “about 250”indicates a range of between 225 and 275. It will be further appreciatedthat the instructions represented by the operations in FIGS. 4, 7 and 10are not required to be performed in the order illustrated, and that allthe processing represented by the operations may not be necessary topractice the invention.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

1. A method of cleaning a substrate comprising: applying an RF signal ata frequency f to a transducer, the transducer being oriented toward thesubstrate such that the transducer emits an acoustic energy at thefrequency f toward the substrate, wherein the substrate and thetransducer are submerged in a cleaning solution; moving the substraterelative to the transducer; and dynamically adjusting the RF signal tomaintain a resonance of the emitted acoustic energy between thesubstrate and the transducer.
 2. The method of claim 1, whereindynamically adjusting frequency f includes automatically adjusting thefrequency f for each cycle of the RF signal.
 3. The method of claim 1,wherein moving the substrate relative to the transducer includesrotating the substrate.
 4. The method of claim 1, wherein dynamicallyadjusting the RF signal to maintain a resonance of the emitted acousticenergy between the substrate and the transducer includes detecting areflected signal corresponding to energy reflected into the transducerexternal from the transducer.
 5. The method of claim 1, wherein thecleaning solution is deionized water.
 6. The method of claim 1, whereinthe cleaning solution includes one or more of a plurality of cleaningchemicals.
 7. The method of claim 1, wherein dynamically adjusting theRF signal to maintain the resonance of the acoustic energy includesdynamically adjusting a frequency f of the RF signal applied to thetransducer.
 8. The method of claim 1, wherein moving the substraterelative to the transducer includes moving the substrate a variabledistance d relative to the transducer.
 9. The method of claim 8, whereinthe distance d varies about ½ wavelength of the frequency f of the RFsignal.
 10. The method of claim 1, further comprising coupling afeedback signal to an input of the transducer and wherein the feedbacksignal includes a reflected signal corresponding to energy reflectedinto the transducer external from the transducer.
 11. The method ofclaim 1, wherein the transducer includes two or more transducers.